Officials in one of the country's most conservative states vowed to appeal the decision and to ask for an emergency stay, the Deseret News reported. The state Attorney General's Office said it filed a notice of appeal Friday but the request for a stay would take several days.
"The federal district court's ruling that same-sex marriage is a fundamental right has never been established in any previous case in the 10th Circuit," spokesman Ryan Bruckman said in a statement.
In Salt Lake County, District Attorney Sim Gill said the county clerk could start issuing marriage licenses immediately. About 120 couples sought them, with some getting County Clerk Sherrie Swenson to perform the ceremony immediately.
Gov. Gary Herbert, a Republican, promised to "defend traditional marriage within the borders of Utah."
"I am very disappointed an activist federal judge is attempting to override the will of the people of Utah," he said.
U.S. District Court Judge Robert J. Shelby, ruling in a lawsuit brought by two gay men, said the state law violates their right to equal protection and due process under the U.S. Constitution.
"The state's current laws deny its gay and lesbian citizens their fundamental right to marry and, in so doing, demean the dignity of these same-sex couples for no rational reason," Shelby found. "Accordingly, the court finds that these laws are unconstitutional."
Moudi Sbeity, a co-plaintiff in the lawsuit with his partner, Derek L. Kitchen, told the Salt Lake Tribune he hopes the two can get married "soon, if all goes well and the state doesn't appeal."
Shelby wrote in his opinion regulation of marriage has traditionally been left to states, but states are obligated to follow the U.S. Constitution.
"The issue the court must address in this case is not who should define marriage, but the narrow question of whether Utah's current definition of marriage is permissible under the Constitution," he wrote.
Utah outlawed same-sex marriage in a 2004 referendum supported by 66 percent of those who voted on it. The judge said "the legal issues presented in this lawsuit do not depend on whether Utah's laws were the result of its legislature or a referendum, or whether the laws passed by the widest or smallest of margins."